Presently, various wireless devices are widely used throughout the world, such that the number of service types is rapidly increasing. Assuming that past radio data transmission is achieved on the basis of audio data (especially, voice data), current radio data transmission has been developed in various formats such as audio, video, photo, and document transmission forms, such that a data transfer rate is exponentially increased. As such, wireless communication standards for providing a higher transfer rate have recently been proposed. For example, LTE can communicate with another party at a higher speed (i.e., a maximum transfer rate of 1 GB/s) that is faster than that of HSDPA by twelve times or greater, using OFDM and MIMO technologies. However, a maximum speed of wireless communication standards capable of transmitting data at high speed can be achieved on the assumption of ideal environmental factors such as speed, channel environment, time/frequency allocation, etc. A data transfer rate capable of being actually experienced by a user is substantially lower than the maximum speed of the wireless communication standards. Specifically, performance of a wireless communication device considering a wireless channel environment is greatly influenced by a channel environment between transceiver devices of signals. Representative examples may be the presence or absence of an obstacle, the distribution of obstacles, device movement speed, etc. In case of using MIMO technology, as one important technology capable of improving data transfer rate, device restrictions caused by the design and arrangement of antennas may affect the MIMO technology.
Under various environmental and physical restricted situations, a repeater capable of amplifying RF signals may be used to compensate for performance deterioration of wireless devices. In case of using a general RF repeater, a method for receiving an RF signal of a wireless device, amplifying a signal including noise and interference, and retransmitting the amplified signal has been used.
FIG. 1 is a conceptual diagram illustrating a general RF signal amplifier. In case of a typical RF signal amplifier shown in FIG. 1, if antennas are installed irrespective of the position and category of antennas of the wireless device, much power emitted from the wireless device may be lost during the input process of a signal amplifier. That is, although the signal amplifier amplifies a signal, not only the signal but also noise is amplified such that it is impossible to obtain maximum performance of the amplifier. In addition, if MIMO technology is applied to the legacy RF signal amplifier, power emitted from each antenna of the wireless device generates mutual interference at a receiver of the RF signal amplifier, and an output unit of the RF signal amplifier amplifies such interference, resulting in deterioration of RF signal amplifier performance.
FIG. 2 shows the relationship between devices configured to use general signal amplifiers that amplify signals of a source device through a signal amplifier and transmit the amplified signals to a destination device, or amplify weak signals of the destination device and transmit the amplified signals. A detailed description of a signal model of a transceiver system including a signal amplifier is shown in FIG. 3.
FIG. 3 shows a general model of a channel model of a legacy MIMO signal amplifier and a simplified channel model of the general model. In this case, on the assumption of user equipment (UE) uplink (UL) transmission (e.g., data transmission from UE to BS (Base Station)/AP (Access Point), N is the number of Tx antennas of a source device, L is the number of Rx antennas with respect to a source device , K is the number of Tx antennas with respect to destination, and M is the number of destination Rx antennas (the relationship between transmission and reception on downlink (DL) transmission is opposite to the relationship between transmission and reception on uplink transmission.)
Referring to a signal amplifier model in a general MIMO channel environment, the signal amplifier receives signals passing through (N×L) RF channels from a source device. Therefore, a reception (Rx) signal is a signal including antenna interference and thermal noise of a device, and the Rx signal is amplified and retransmitted so that undesired signals are unavoidably amplified. In order to solve the noise amplification problem, a distance between a source device and a signal amplifier can be minimized. For example, the user equipment (UE) may be mounted to the signal amplifier. In this case, the above-mentioned noise amplification problem can be minimized from the viewpoint of uplink. In addition, from the viewpoint of downlink, the signal amplifier receives improved quality signals from the base station (BS) using a high-performance signal amplifier Rx antenna, and signal loss of the corresponding signal is minimized and transmitted to the UE, resulting in improved DL performance.
However, if the distance between the UE and the signal amplifier is very short, performance sensitivity is greatly increased in response to a signal amplifier antenna acting as a receiving antenna and a UE antenna acting as a mobile antenna. For example, a beam pattern of Mobile Antenna #1 is well matched to that of Receiving Antenna #1 in FIG. 4(a), so that a high SINR (Signal to Interference plus Noise Ratio) appears. In contrast, a beam pattern of Mobile Antenna #4 is not relatively well matched to that of Receiving Antenna #4, so that a relatively low SINR appears. If a UE is composed of multiple antennas (N>1), performance sensitivity may encounter more serious problems, because SINRs between UE antennas are changed in response to a relative position of signal amplifier antennas and mutual interference may occur between antennas. For example, it is assumed that, although Mobile Antenna #1 is well matched to Receiving Antenna #1, Mobile Antenna #2 is improperly matched to Receiving Antenna #2. In this case, a difference in channel gain (or pathloss) between UE Rx antennas increases from the viewpoint of downlink, so that it is difficult to simultaneously transmit data through multiple streams or layers (i.e., it is difficult to achieve high rank transmission implemented with multiple Rx antennas), resulting in reduction of spatial diversity. That is, the reception stability increasing effect obtained by combining Rx signals of a plurality of Rx antennas having different channel characteristics is also reduced. Similarly, since different channel gains are generated between UE Tx antennas from the viewpoint of uplink, the probability that data is to be transmitted using a high rank is reduced and the transmit diversity effect is also reduced. Therefore, a method for minimizing signal attenuation between a wireless device and a signal amplifier and optimizing MIMO performance has been proposed. Specifically, a method for converting a channel between the signal amplifier and the wireless device into a parallel SISO channel through antenna and frequency characteristics of a wireless device and associated adaptive link formation and optimizing performance of the signal amplifier has recently been proposed.